1. Field of the Invention
The present invention relates to a stacked piezoelectric element in which layers of piezoelectric material are stacked, and a vibration wave driving apparatus using the stacked piezoelectric element.
2. Description of the Related Art
Conventionally, piezoelectric material, which is a typical material having electromechanical energy conversion functions, is used as piezoelectric elements in various applications. Recently, in particular, stacked piezoelectric elements in which plurality of layers are stacked, integrally formed and sintered are commonly used. The stacked piezoelectric element provides greater deformation strains and greater power from lower voltage through layer stacking as compared to a single-layer piezoelectric element, and also enables downsizing by reducing the thickness of each of the stacked layers.
Generally, a stacked piezoelectric element comprises piezoelectric layers respectively formed of a plurality of layers of piezoelectric material consisting of piezoelectric ceramics, and electrode layers, which are conductive layers, arranged adjacent to each piezoelectric layer and formed from conductive material. Pluralities of piezoelectric layers and electrode layers are stacked upon each other to form multilayer stacking and sintered. Subsequently, polarization is performed thereon to provide the entire stacked piezoelectric element with piezoelectricity.
FIG. 9 is a structural diagram of a conventional stacked piezoelectric element disclosed in Japanese Laid-Open Patent Publication (Kokai) No. 2005-168281.
A stacked piezoelectric element 20 is a vibrating body used in a linearly-driving vibration wave motor, and comprises a driving unit 26 that applies voltage to provide drive and generate vibration, and a non-driving unit 27 that does not apply voltage. The non-driving unit 27 has a thickness that allows generation of bend vibrations.
The non-driving unit 27 comprises a plurality of layers from a first layer to a predetermined layer (for instance, a twentieth layer). Each layer is comprised of a piezoelectric layer 22a that does not have an electrode layer. The driving unit 26 comprises a plurality of layers from a predetermined layer (for instance, a twenty-first layer) to a last layer (for instance, a thirtieth layer), and is configured so that piezoelectric layers 22b on which bisected electrode layers 23-1 and 23-2 are formed and piezoelectric layers 22c on which non-bisected electrode layers 23-3 are formed are alternately stacked upon each other to form multilayer stacking.
The driving unit 26 and the non-driving unit 27 are simultaneously stacked together and fired to configure the stacked piezoelectric element 20. The non-driving unit 27 is set to have a thickness that enables generation of effective bend vibrations at the stacked piezoelectric element 20. If the non-driving unit 27 is too thin, vibration energy generated by the driving unit 26 cannot be extracted as bend vibrations sufficient for driving.
The respective piezoelectric layers 22b having the bisected electrode layers 23-1 and 23-2, and the respective piezoelectric layers 22c having the electrode layers 23-3 that substantially cover the entire surface of the respective piezoelectric layers 22c, are independently electrically connected via throughholes 24-1, 24-2 and 24-3, and are electrically conductive with three surface electrode layers 25 arranged on the surface of the bottommost piezoelectric layer. A throughhole is a hole penetrating the piezoelectric layers 22, which is filled with conductive material. The piezoelectric layers 22b and 22c respectively sandwiched between the electrode layers 23-1, 23-2 and 23-3 are provided with predetermined polarities.
After provided with the above-mentioned polarities, the surface electrode layers 25 at the bottommost face of the stacked piezoelectric element 20 are scraped off by lapping, and a flexible circuit board is adhered to a predetermined position on the surface to enable the stacked piezoelectric element 20 to connect with a driving circuit. Then, by grounding the electrode layer 23-3 of the stacked piezoelectric element 20 and applying a high-frequency voltage having a temporal phase difference to the electrode layers 23-1 and 23-2, two different bend vibrations may be simultaneously generated.
On the other hand, wavinesses and warping due to contraction during firing are likely to occur on a piezoelectric actuator substrate formed of a thin plate-like stacked body. In this light, as shown in FIG. 10, there is disclosed in Japanese Laid-Open Patent Publication (Kokai) No. 2004-349688 discloses that high-contraction layers 31 consisting primarily of two layers of conductive material are arranged between each ceramic layer 32 to be axisymmetrical with respect to an imaginary line 34 which passes through a position at one-half thickness of the stacked body and which is parallel to the surface of a piezoelectric actuator substrate 30. Furthermore, as shown in FIG. 11, Japanese Laid-Open Patent Publication (Kokai) No. 2004-349688 also describes high-contraction layers 31 axisymmetrically arranged with respect to the imaginary line 34 in a piezoelectric actuator substrate 33 having three or more ceramic layers 32.
As described in above-described Japanese Laid-Open Patent Publication (Kokai) No. 2005-168281, in the stacked piezoelectric element 20 in which coexist the driving unit 26 having the electrode layers 23-1, 23-2 and 23-3 and the non-driving unit 27 having no electrode layers, during firing, the electrode layers 23-1, 23-2 and 23-3 consisting of conductive material contract earlier than the piezoelectric layers 22a, 22b and 22c consisting of piezoelectric material and are therefore more likely to develop warping and other deformation. In this light, as described in Japanese Laid-Open Patent Publication (Kokai) No. 2004-349688, axisymmetrically arranging the plurality of high-contraction layers 31 consisting primarily of conductive material with respect to the imaginary line 34 of the stacked body is evidently effective in reducing warping.
However, in the stacked piezoelectric element 20 described in Japanese Laid-Open Patent Publication (Kokai) No. 2005-168281, the driving unit 26 has a plurality of electrode layers 23-1, 23-2 and 23-3. In addition, a polarizing surface electrode layer 25 simultaneously fired with the stacked piezoelectric element 20 is provided on the surface of the driving unit 26, and the piezoelectric layers 22a, 22b and 22c are polarized. Since the plurality of polarized piezoelectric layers 22a, 22b and 22c contract in a surface direction in the same manner as the contraction of the electrode layers 23-1, 23-2 and 23-3 during firing, the degree of warping will be further increased. Therefore, merely arranging the plurality of high-contraction layers 31 as described in Japanese Laid-Open Patent Publication (Kokai) No. 2004-349688 is insufficient for suppressing warping.
The existence of such warping means that even when planarizing the surface of the stacked piezoelectric element by performing double-side lapping or grinding, the internal electrode layers will remain warped with respect to the processed planarized surface, and in extreme cases, such warping results in the internal electrode layers exposed on the processed surface.